Therapeutic synergism between methotrexate (MTX) and 1-beta-D-arabinofuranosylcytosine (ara-C) appears related in part to an inhibition by MTX of the increase in dCTP that accompanies the treatment of L1210 cells with ara-C alone. Previous work indicated that this effect by MTX on the cellular pools of dCTP was not mediated via MTX-induced decreases in the levels of dTTP, when both drugs were administered to mice with L1210 leukemia. On the other hand, these studies, as well as the earlier survival studies with L1210 leukemia, suggest that concurrent treatment with ara-C modulates the oncolytic action of MTX. The biochemical effects of ara-C on the pharmacologic action of MTX will be investigated for possible leads to the postulated additional site of oncolytic interaction between these drugs. These investigations will be initiated with studies of the inhibition of thymidylate synthetase by ara-C and with the expectation that an understanding of the effect by ara-C on this enzyme will indicate the biochemical basis for the changes in the cellular pools of dCTP when both drugs are administered. 4'-Demethylepipodophyllotoxin-9-(4,6-0-2-thenylidene-beta-D-glucopyranoside) (VM-26) potentiates the oncolytic action of ara-C on L1210 leukemia. With L1210 cells in culture, VM-26 increased the conversion of ara-C to ara-CTP. Studies are in progress to determine the effect of ara-C on the repair of lesions in DNA that are produced by VM-26 and the effect of VM-26 on the incorporation of ara-C into DNA. Subsequent studies will investigate the nature of the lesion produced by VM-26 in DNA and whether the cell participates in the formation of the single strand breaks that are observed after the treatment of cells with VM-26 and the sedimentation of their DNA on alkaline sucrose gradients.